|The distribution of fossils across the continents is one|
line of evidence pointing to the existence of Pangaea.
The outer layers of the Earth are divided into the lithosphere and asthenosphere. This is based on differences in mechanical properties and in the method for the transfer of heat.
The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken up into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates.
|The tectonic plates of the world were mapped in the second half of the 20th century.|
The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes, though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents.
The shapes of continents such as eastern South America and western Africa would fit neatly if pushed together. The discovery of matching fossils and rock layers on land separated by wide oceans provided further evidence that landmasses were once united. Scientists call this supercontinent Pangaea. The slow movement of Earth’s plates caused Pangaea to split apart.
Tectonic plates are able to move because the Earth's lithosphere has greater strength than the underlying asthenosphere.
Plate motions range up to a typical 10–40 mm/year (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/year (Nazca Plate; about as fast as hair grows).
The location where two plates meet is called a plate boundary.
Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries.
Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches.
The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and widely known today.
Image showing the three main tectonic plates boundary
types and associated volcanoes and earthquakes